1. Field of the Invention
The present invention generally relates to a disc drive device for driving at least one magnetic storage disc and, particularly, to a brushless motor assembly used in the disc drive device. More particularly, the present invention relates to the brushless motor assembly of a type including a rotor magnet mounted internally of a disc carrier hub for the support of the magnetic storage disc.
2. Description of the Prior Art
With increase in both the handling capacity and the speed of computers, the magnetic storage disc drive device has come to have its operating speed increased from 3,600 rpm to 5,400 rpm and, then, from 5,400 rpm to 6,300 rpm, and yet, the magnetic storage disc drive device operable at a higher speed of, for example, 7,200 rpm is currently developed. On the other hand, disc drive devices for use with 3.5-inch magnetic storage discs have superseded those for use with 8-inch magnetic storage discs and are now likely to supersede those for use with 5-inch magnetic storage discs. Also, to increase the storage capacity, the use has come to be made of a disc array and an MR head. Particularly in the disc drive device utilizing the MR head is employed, the head-to-disc gap is not greater than 0.1 .mu.m when the drive device is in operation with its reliability likely to have reached a limit. For this reason, the design of a disc drive device is required to meet severe requirements such as, for example, minimization of dust from a drive motor used, suppression of grease scattering and outgassing.
The drive motor assembly used in the prior art disc drive device will now be described with reference to FIGS. 5 and 6 which show a longitudinal sectional view and an enlarged longitudinal sectional view of an upper portion of the prior art disc drive device, respectively.
Referring to FIGS. 5 and 6, the prior art disc drive device comprises a generally rectangular cup-like casing having a base plate 28 and opening upwardly and a cover plate 27 closing the top opening of the casing to thereby define a disc chamber 36. The disc drive device shown therein includes a fixed shaft 1 extending between the cover plate 27 and the base plate 28 and having upper and lower bearings 7 and 8 mounted thereon and spaced a distance from each other by means of a spacer sleeve 12. A generally cylindrical hub 2 for the support of a plurality of, for example, six, magnetic storage discs 33 is mounted around the fixed shaft 1 through the upper and lower bearings 7 and 8. The cylindrical hub 2 includes a corresponding number of disc spacers 34 mounted therearound and held in position by means of an annular clamping plate 35 that is secured to an upper end of the cylindrical hub 2 by means of a plurality of set screws 31. Although not shown, the disc drive device also includes magnetic read-write heads one for each of opposite surfaces of each of the magnetic storage discs 33, said read-write heads being supported for movement between retracted and loaded positions in a direction radially of any one of the magnetic storage discs 33.
A generally disc-shaped bracket 3 is fixedly mounted on the base plate 28 by means of a plurality of set screws 30 and has a central portion fixedly supporting a lower end of the fixed shaft 1. The cover plate 27 closing the top opening of the cup-like casing is secured to an upper end of the fixed shaft 1 by means of a set screw 32 with the magnetic storage discs 33 consequently sealed within the casing.
A cylindrical rotor magnet 4 is fixedly secured to an inner surface of a lower end portion of the cylindrical hub 2 for rotation together with the cylindrical hub 2, and a stator core 6 including a coiled winding 5 is positioned radially inwardly of the rotor magnet 4. This stator core 6 is fixedly mounted on a lower holder 13 which is in turn fixedly mounted on a lower end portion of the fixed shaft 1 below the lower bearing 8.
By the presence of a labyrinth seal means effective to reduce a radial gap between the lower holder 13 and the hub 2 and, also, a radial gap between the lower holder 13 and the lower bearing 8, any possible scattering of lubricant grease from the upper and lower bearings 7 and 8 is prevented with the disc chamber 36 consequently kept clean.
Reference numeral 10 represents a biasing spring mounted around the fixed shaft 1 and interposed between the lower bearing 8 and the lower holder 13 for applying a biasing force to the upper and lower bearings 7 and 8.
Fixedly mounted on the bracket 3 and positioned immediately below the rotor magnet 4 and the stator core 6 is a stator substrate 9. This stator substrate 9 has a position detecting element 15 mounted thereon for controlling excitation of the coiled winding 5.
As best shown in FIG. 6, an upper magnetic fluid seal 11 mounted on a seal holder 14 that is fixedly positioned inside the cylindrical hub 2 and above the upper bearing 7 retains a magnetic fluid 16 between it and the fixed shaft 1 to thereby avoid scattering of the lubricant grease from the upper and lower bearings 7 and 8. To fix the magnetic fluid seal 11 in position as mounted on the seal holder 14, a bonding material is deposited at 19 to tightly secure the magnetic fluid seal 11 to the seal holder 14. The bonding material 19 referred to above may preferably be UV-curable resin or thermosetting resin (including epoxy resin). A magnetic fluid protective plate 17 for preventing any possible scattering of the magnetic fluid is mounted on and secured by means of a bonding material, 18 which may be the same as the bonding material 19, to the seal holder 14 so as to overhang the magnetic fluid seal 11.
The cleanness of the motor assembly so constructed as hereinabove described will now be discussed. A splash of grease from the upper and lower bearings 7 and 8 is reduced or shut off by the labyrinth seal means effective to reduce the radial gap between the lower holder 13 and the hub 2 and, also, the radial gap between the lower holder 13 and the lower bearing 8 and the disc chamber 36 is therefore kept clean.
It has, however, been found that the prior art disc drive device has a problem. Since the scattering of the lubricant grease increases with an increase in number of revolutions of the motor used in the drive device, the grease tends to leak from the motor bearings into the disc chamber, where the storage discs are accommodated, when the motor is driven at a high speed. Considering that the cleanness is an ultimate requirement due to the extremely reduced head-to-disc gap, even the slightest leak of the lubricant into the disc chamber results in a head crush or an erroneous information recording or reading. In addition, the prior art disc drive device makes use of a substantial mount of bonding material which poses a problem associated with outgassing. Once this outgassing occurs, the magnetic storage discs may be eroded to such an extent as to eventually result in an occurrence of the head crush.
The use of the substantial amount of the bonding material makes it difficult or complicated to assemble the disc drive device, resulting in increase of the manufacturing cost.